1. This study was designed to test the hypothesis that food reductions caused by forest spraying with Lepidoptera-specific insecticides would adversely affect forestdwelling songbirds.
2. The study was conducted in two 300 ha spray blocks, one treated with Bacillus thuringiensis (Bt) at 30 BlUs per ha and the other with tebufenozide (MIMIC) at 70 g per ha, and in an untreated control area.
3. In the tebufenozide block, lepidopteran numbers were reduced by 96% on spruce and by 77% on aspen, relative to the control. The Bt application had little effect on the lepidopteran fauna of aspen (15% reduction) and no effect on spruce.
4. Because of the poor efficacy of the Bt treatment, any conclusions regarding indirect effects of Bt on forest songbirds would be meaningless.
5. Following treatment, Bay-breasted and Tennessee Warblers spent a greater proportion of their time in the budworm host trees (spruce and fir) in the tebufenozide block vs. the control block. This change may reflect a need for increased foraging effort to find prey items which were reduced in numbers by the spray. The tebufenozide treatment had no effect on foraging manoeuvers or choice of location (height) in the trees.
6. Bay-breasted, Tennessee and Cape May Warbler diets and body lipid levels were unaffected by the tebufenozide application.
7. The tebufenozide treatment had no effect on Tennessee Warbler nest success, nestling diet or nestling growth rate. Following treatment, female Tennessee Warblers spent less time brooding young, and their foraging trips away from the nest were longer in duration, in the tebufenozide block vs. the control block.